Rubber goods such as tire treads often are made from elastomeric compositions that contain one or more reinforcing materials such as, for example, particulate carbon black and silica; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
Good traction and resistance to abrasion are primary considerations for tire treads; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat build-up during operation of the tire. These considerations are, to a great extent, competing and somewhat contradictory: treads made from compositions designed to provide good road traction usually exhibit increased rolling resistance and vice versa.
Filler(s), polymer(s), and additives typically are chosen so as to provide an acceptable compromise or balance of these properties. Ensuring that reinforcing filler(s) are well dispersed throughout the elastomeric material(s) both enhances processability and acts to improve physical properties. Dispersion of fillers can be improved by increasing their interaction with the elastomer(s). Examples of efforts of this type include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, and surface grafting.
The section of a polymer chain from the site of the last crosslink to an end of the polymer chain is a major source of hysteretic losses; this free end is not tied to the macro-molecular network and thus cannot be involved in an efficient elastic recovery process and, as a result, energy transmitted to this section of the polymer (and vulcanizate in which such polymer is incorporated) is lost as heat. Ensuring that these polymer chain ends are tied to, or otherwise interact well with, reinforcing particulate fillers, is important to many vulcanizate physical properties such as, for example, reduced hysteresis. Chemically modifying the polymer, typically at a terminus thereof, is one of the most effective ways of increasing interactivity of fillers and polymers.
Certain tests have come to be recognized as correlating certain physical properties of vulcanizates with performance of products, particularly tire treads, made therefrom. For example, reductions in hysteresis (heat build-up during operation) have been found to correlate with higher rebound values and lower loss tangent values (tan δ) at high temperature, better handling performance often correlates with higher elastic modulus values at high temperature and strain, ice traction has been found to correlate with lower modulus values at low temperatures, etc. (In the foregoing, “high temperature” typically is considered to be about 50°-65° C. while “low temperature” is considered to be about 0° to −25° C.)
Various elastomeric materials often are used in the manufacture of vulcanizates such as, e.g., tire components. In addition to natural rubber, some of the most commonly employed include high-cis polybutadiene, often made by processes employing catalysts, and substantially random styrene/butadiene interpolymers, often made by processes employing anionic initiators. Functionalities that can be incorporated into high-cis polybutadiene often cannot be incorporated into anionically initiated styrene/butadiene interpolymers and vice versa.